Automotive fuel tanks are often provided with float-type vapor vent valves, which selectively vent fuel vapor from the tank to a vapor treatment apparatus such as a carbon canister. To prevent flooding of the carbon canister with liquid fuel, vent valves are often controlled by a spring-assisted float member which rises and falls with liquid fuel levels to selectively open and close the valve.
These float members are frequently "balanced" relative to the density of liquid fuel by adjusting 1) float density, and 2) the spring assist force on the float member. Floats accordingly may have a density heavier than fuel, the same as fuel, or lighter than fuel, with an appropriate spring assist so that the desired responsiveness is obtained when the float is submerged. This process can be complicated by the need to have the float react appropriately in both upright and rollover conditions.
While the density of the float is fairly easy to control during the manufacturing process, the relatively weak springs used in such valves has made them difficult to calibrate. Thin wire springs in a production lot can vary sufficiently to create calibration and operating problems for the valves in which they are installed. Given the importance of their protective fuel-controlling function, calibration of individual valves is often needed. Typically, valves are individually calibrated by pre-stretching or compressing individual springs until the proper calibration is achieved, or trimming them to length on an individual basis. Such prior art methods are slow, inefficient, and inexact, and are therefore subject to improvement.